Recovery of lower aliphatic acids from their salts



Patented May 26, 1942 aeooven'r oF Lo ER surname Aoins FROM THEIR SALTS Henry Dreyfus, London, and Walter Henry Groombridge, Spondon, near Derby, England,

assignors to Celanese Corporation of America, a corporation of Delaware 'No'Drawingf Application-illecember 27,1940, Se-

rial No; 372,021; In Great BritainJanuary 2 lsciaims. ((11. 250-541) This invention relates to the production of fatty acids'from'their salts.

In many industrial processes there are obtained solutions of metal salts of the lower fatty acids, especially of acetic acid. For example the waste liquor obtained after sapom'fying cellulose acetate textile materials may comprise sodium acetate or potassium acetate. It is an object of the present invention to obtain the free fatty acid from solutions of its salts. 1

In U. S. application's. No. 263,420, filed March 22, 1939, we have described a process-in which carbon dioxide is brought into contact with a solutionof a metal salt of a'fatty acid in concentrated aqueous ethyl alcohol maintained at a temperature above 70 C. so as to form an acid salt of the metal from which the free acid may readily be obtained. The reaction between carbon dioxide and sodium acetate, for example, re-v sults in the precipitation of sodium bicarbonate and the formation of acid sodium acetatefwhich remains in solution.- 7

'It is stated in the said application that the solution so obtained may be freed from sodium bicarbonate, for example by decantation, filtration or centrifuging, preferably in the cold and may then be treated to recover acetic acid from the acid acetate, for example by heating it under a fractionating column whereby a high proportion of the alcohol may be recovered substantially or nearly free from acetic acid and the residue, by being heated more strongly, caused to yield its loosely combined acetic acid.

It has now been found that by carrying out such a process under a high pressure of carbon dioxide, it is possible to form a solution containing the acid salt in such high concentration that it can be separated therefrom by crystallisation, thus avoiding the necessity of evaporating the solvent alcohol from the solution, and hence improving considerably the economics of the process.

According to the invention therefore, carbon dioxide is brought into contact with a solution of a metal salt of a fatty acid in a concentrated aqueous alcohol so as to form an acid salt of the metal from which the free acid may readily be,

obtained, the carbon dioxide being employed at such a pressure that the concentration of the acid acetate produced in thesolvent is in excess of its solubility therein at about atmospheric temperature, and acid salt is separated from the solution by crystallisation.

The pressure of carbon dioxide employed may,

in general, be in excessof .400 lbs. per. square. inch,

and pressures of the order of 500 lbs. per square inch give very satisfactory results, although pressures upto 800 lbs. per square inch or more may beused, if desired. Naturally the concentration of the metal salt solution employed as starting material, and the temperature used in the process, are factors which influence the conversion to acid salt andhence the concentration of the latter in the solution obtained. The concentration of the .metal salt solution is preferably high, and although not necessarily high enough to saturate the solvent alcohol at room temperatures, may be such that cooling of the solution a few degrees below room temperatures causes crystallisation of .ably higher, e. g. 90-98%.

serves only as a vehicle for the water and as solvent for the acetate or other fatty acid salt it the salt. The reaction temperatures may most suitably be from -130 C., and the use of temperatures of this order, while being favourable to the reaction and making possible the production of a solution from which the acid salt can be crystallised by cooling, offers the additional advantage that it assists the precipitation of the bicarbonates produced in a form in which they are readily removable by filtration.

The alcohol used in the process should contain an amount of water at least stoichiometrically equivalent to the amount of acetate or other fatty acid salt in solution. By employing alcohol of high concentration full advantage can be taken of the insolubility of the bicarbonates in alcohol a and it hasbeen found preferable to use ethyl alcohol of at least concentration and prefer- Since the alcohol may be replaced wholly or in part by other alcohols and in part, by other organic liquids including hydrocarbons, e. g. benzene and parafiins and esters, e. g. methyl and ethyl acetate, so long as be collected in the space between the draw-off and the bottom of the tower, this space thus forming a settling compartment for the precipitate.

The settling compartment, which may withadvantage. be wider thanthe body of the tower, may

the precipitate and returned to theprocess, The

rest of the solution after leaving the .tower may be filtered or otherwise treated to remove any bicarbonate it contains, and is then cooledtoseffect crystallisation of the acid salt.

After the crystallisation step, which may be carried out simply by cooling to apredetermined temperature or by cooling in stages with sepa-. 1

ration of each crop of crystals produced, the mother liquor may be returned to the process as solvent for a further quantity ofaithe' metal salt used as starting material, and whenthis procedure is adopted it is, of course, unnecessary to attempt to separate as muchv as possible of the acid salt, for'instance by strong cooling during carbonate is readily separable from the solution by filtration. I

The following example illustrates the invention as applied to the production of acetic acid fr m sodium acetate:

" Example autoclave provided with a stirrer or with ;means for effecting agitation of the autoclave is charged with asolution of sodium acetate in ethyl alcohol of 92-94% concentration, the solution containing foreach litre'of alcohol about 250 grams ofsodium acetate. Carbon dioxide is introduced into the autoclave under a pressure,

of 500'lbs.per square-inch and the autoclave [is heated to a temperature of 80 C. After 3 crystallisation. Indeed, strong cooling of the solution is liable to cause or stallisation of the I y I under reduced pressure and after a little alcohol and ethyl acetate is distilled pure acetic acid comes over leaving a residue of sodium acetate which is also returned to the process.

I Subsequentbatches in-the process can be made normal salt together with the acid salt, thus adding to the bulk of the salts to be heated to obtain the free acid, and-consequently to the heat needed in theprocess, without producing a corresponding increase in the quantity of acid produced. Preferably, therefore, the crystallisation is effected by cooling in one or more stages to about room temperature or even to a temperature a little above this, e. g. 20 C., whereafter' the acid salt obtained may be freed from mother liquor by filtration, centrifuging-or other operation, and decomposed to produce the normal salt and free acid, the former being returned to the process.

The metal bicarbonate produced can be used in any suitable way. Advantageously sodium or potassium bicarbonate may be heated to liberate carbon dioxide, preferably under the pressure employed in the process, and this carbon dioxide returned 'to the fatty acid salt solution. The carbonate remaining may, if desired, be converted into the corresponding hydroxide.

The invention is thus of particular interest in" the saponificationo f cellulose acetate textile materials with caustic soda or potash, since it hours the bicarbonate precipitated is filtered off from the product and the clear liquor cooled to a temperature of about 20 C. Acid sodium acetate crystallises out and is separated from the mother liquor which is returned to the process.

The crystalline acid sodium acetate is distilled up from the mother liquor obtained from the. crystallisation of the acid sodium acetate and sodium acetatei left after distillation of the acid salt together with sufficient-sodium acetate to make up for that from which acetic acid has been recovered and alittle alcohol to replace that left on the bicarbonate and acid acetate removed provides a simple and cheap cyclic processwhereby substantially all the sodium or potassium base can be regenerated for re-use, and the acetic acid removed from the materials can be recovered. If desired, sodium acetate may be converted into potassium acetate before the treatment, so as to make use of the greater solu-1 bility of potassium acetate in aqueous alcohol.

For example, potassium chloride may be added'to an alcoholic solution of sodium acetate, whereby sodium chloride is precipitated, leaving potassium acetate in the solution: the proportion of sodium precipitated as chloride may be increased by evaporating off alcohol, preferably until a saturated solution of potassium acetate remains. v

Although the invention has" been described with particular reference. to thetreatment of" breaks down .into:.the normal carbonate,. setting free carbon dioxide for re-use. "The calcium from the process. A suitable mixture is one containing for each litre of alcohol 200 grams. of normal sodium acetate and about grams. of acid sodium acetate.

Having described our invention, what we desire to securebyLetters Patent is:

1. Process for the manufacture of lower aliphatic acids from their salts, which :comprises bringing. carbon dioxide into contact with a solution of the salt in an aqueous ethyl alcohol of to 98% concentration at an elevated temperature and under high pressure so as to form the corresponding acid salt in a concentration higher than its solubility in the alcohol at atmospheric temperature, separating the acid salt by crystallization and recovering the free acid therefrom by distillation.

2. Process for the manufacture of acetic acid from alkali metal acetates, which comprises bringing carbon dioxide into contact with a solution of the acetate in an aqueous ethyl alcohol of 90 ,to 98% concentration at an elevated temperature-and under high pressure so as to form .tate by crystallization and recovering free acetic acid therefrom by distillation.

3. Process forthe manufacture of lower aliphatic'acids from their salts, which comprises bringing carbon dioxide into contactwith a solution of the salt in an aqueous ethyl alcohol of 90 to 98% concentration at a temperature of from 70 to C. and under a pressure of 400 to 800 pounds per square inch so as to form the corresponding acid salt in a concentration higherthan its solubility in the alcohol at atmospheric terriperature, separating the acid salt by crystallization and recovering the free acid therefrom by distillation.

4. Process for the manufacture of aceticac'id from alkali metal acetates, which comprises bringing carbon dioxide into contact with a solution of the acetate in an aqueous ethyl alcohol of 90 to 98% concentration at a temperature of from 70 to 130 C. and under a pressure of 400 to 800 pounds per square inch so as to form the corresponding acid salt in a concentration higher than its solubility in the alcohol at atmospheric temperature, separating the acid salt by crystallization and recovering the free acid therefrom by distillation.

5. Process for the manufacture of acetic acid from alkali metal acetates, which comprises bringing carbon dioxide into contact with a solu- HENRY DREYFUS. WALTER HENRY GROOMBRIDG'E. 

